Mono from SVN is now able to use LLVM as a backend for code generation in addition to Mono's built-in JIT compiler. "This allows Mono to benefit from all of the compiler optimizations done in LLVM. For example the SciMark score goes from 482 to 610. This extra performance comes at a cost: it consumes more time and more memory to JIT compile using LLVM than using Mono's built-in JIT, so it is not a solution for everyone. Long running desktop applications like Banshee and Gnome-Do want to keep memory usage low and also would most likely not benefit from better code generation. Our own tests show that ASP.NET applications do not seem to benefit very much (but web apps are inherently IO-bound). But computationally intensive applications will definitely benefit from this. Financial and scientific users will surely appreciate this performance boost."

Pre-JITing requires the generated code to be a shared library. This is required so that the same code for say System.XML can be used in two different applications that might end up loading the pre-generated code at different addresses.

The code produced by shared libraries is a bit slower than static code. Invocations need to go through at dispatch table (In Mono and ELF systems it is called the PLT table) and access to global variables also has to go through extra steps and usually ties up a register to keep track of things.

So all of these things mean that you end up with code that might not be as efficient as the static code.